The present invention relates to a furnace for firing wire-like products, and in particular to a vertical furnace for firing copper wire enamelled with a cladding of insulating plastics resin.
As is known, furnaces currently utilized for firing the cladding of copper wires are of the continuous tunnel type and include at least one chamber of elongate form in which the wire is advanced longitudinally. In a first portion of this chamber evaporation of the solvents which impregnate the cladding resin take place; in the second portion of the chamber, maintained at a higher temperature than that of the first portion, polymerization and cross linking of resin take place.
The heating of the said portions, and in particular of the first, is generally achieved by convection, that is to say by introducing a flow of hot gases at an appropriate flow rate and temperature; conveniently such gases are constituted at least in part by combustion products of the solvent vapours which are released in said first portion of the principal chamber, thus obtaining the dual function of lowering the toxicity of these vapours, which are transformed into carbon dioxide and steam, and re-using at least a part of the thermal energy which is generated during their combustion. Furnaces of the known type have several disadvantages.
First of all, the direct introduction of hot gases into the first portion of the principal chamber cannot take place at a very high speed in that it would cause surface non-uniformity (in particular undulations) on the cladding of the wire, making the electrical insulating characteristics of the wire itself irregular. Similarly, these gases cannot be introduced at a very high temperature; it can in fact be experimentally verified that an excessively high temperature, in combination with a relatively high speed of convection, causes surface imperfections on the cladding.
It is evident that this sets precise operational limits on the furnace since the speed of advancement of the wire (which determines the dwell time of each portion of wire in the first portion of the principal chamber) cannot be increased beyond certain limits which guarantee the complete evaporation of the solvents and the surface regularity of the cladding on the wire.
In the case of vertical furnaces, a further disadvantage is constituted by the tendency to suck in external air through the lower inlet opening by the chimney effect; this implies a lowering of the mean temperature within the first portion of the principal chamber and the consequent necessity to provide further thermal energy.